Modern technology has produced a number of useful electronic identification methods and devices. Most familiar are the ubiquitous barcodes and magnetic strips that, together with their readers, are widely employed by businesses and others to perform several identification functions. The main reason barcodes and magnetic strips are so widely used is that they are very cheap.
Barcodes and magnetic strips are limited, however, by the relatively small amount of data they can encode and the effective range at which they can be read, which is quite short. Magnetic strips, for example, generally have such a limited range that the reader must be in direct contact with the strip in order to decode the data thereon. In the very few cases that a magnetic strip is read with a device other than a direct contact reader, the effective reading range is still only a few centimeters at best. Similarly, the effective range at which barcodes can be reliably read is typically not better than a few centimeters.
In addition to range limitations, both barcodes and magnetic strips are impossible to read if there is any obstruction between the reading device and the barcode or magnetic strip. When reading a magnetic strip or barcode, the orientation of the reading device relative to the barcode or magnetic strip also presents a problem. If the reading device is not properly aligned or is held at an incorrect angle, the encoded information cannot be read. As a result of these problems, each individual read operation requires manual scanning by a human operator if high read accuracy is needed. The attractive feature of barcodes and magnetic strips is that they are inexpensive. However, their inherent limitations have prevented their use in a wide range of applications for machine readable text where highly reliable and totally automated reading is required for read ranges of up to several meters.
The radio frequency identification (RFID) tag is another prior art type of identification device. When interrogated by a reading device which is also denoted as interrogator, RFID tags reflect or retransmit a radio frequency signal to return an encoded identification number to the interrogator. A good example of RFID tags is their usage in the collection of highway and bridge tolls. A RFID tag is positioned on a user's vehicle to respond to an interrogation signal when the vehicle passes through a toll collection point. A reading device connected to a computer processes the tag identification number and uses the decoded information to charge a toll to the user by deducting the amount due from the user's credit card or other account.
Prior art RFID tag devices are of two basic types; those that contain a microchip and those that do not. There is a radical difference in cost and performance between these two types; to such an extent, in fact, that they rarely compete with one another as to the appropriate type of use. As a general rule, chipped tags cost more but have a larger data capacity than chipless tags. Chipped tags, for example, are usually not available below a unit cost of about $1 each when ordered in a quantity of less than 1 million; whereas many chipless tags are projected to cost less than 20 cents each, even when manufactured in quantities of 100,000.
Most RFID tags will have a longer reliable range than magnetic strips and barcodes. As a rule, RFID tags can be interrogated without having a significant line of sight and orientation problems as are evidenced by barcodes and magnetic strips. Although chipped tags do have a longer range than magnetic strip and barcode systems, the range at which they can be reliably used is still a limiting factor.
Chipped tags are by far the most popular. A chipped tag consists of four elements or features: a computer microchip, circuits for converting radio signals to computer data signals and back to radio signals, an antenna, and a means for providing DC power to the chip circuitry. In low cost RFID chip tags, the first two features are often partially or totally integrated into a single microchip, which integration requires certain compromises in tag performance (read range, number of bits, etc). This combination of features also leads to certain integrated circuit (IC) cost and/or design compromises to accommodate both digital and radio frequency circuitry on a single IC. The impact of these design compromises can be partially compensated for by use of low radio frequency (RF) operating frequencies that, in turn, lead to rather large and expensive antennas.
The most daunting problem with chipped tags is the need for DC power for the chip circuitry. The combination of environmental issues coupled with severe constraints on costs, size and weight usually requires that the tag not have a battery or other onboard power source. The only generally useable solution is to obtain DC power by converting RF power received from the tag reader signal into DC power within the tag. Those skilled in the pertinent art term tags without a battery or other power source as passive tags, while those that contain a battery or other source are termed as active tags. The passive method of providing DC power to a chipped tag requires a more efficient tag antenna and higher transmitted power levels from the reader. It also requires added components which will either add to the cost of the microchip or to the cost of the tag for the required extra electrical components in the tag, which will also result in an increased tag size. The most important limitation of passive powered chip tags, however, is the severe restriction on the read range of the tag because a signal that is sufficiently strong to power the tag only extends a short distance from the tag reader antenna. Thus, while chipped tags have the dominant share of the RFID market, the high cost and limited read range combine to prevent chipped tags from replacing either barcodes or magnetic strips in any significant manner.
Chipless RFID tags do not contain a microchip but instead, rely on magnetic materials or transistorless thin film circuits to store data. A major advantage of chipless RFID tags is their relatively low cost.
Chipless RFID tags have the disadvantage that they can be read out by any interrogator that uses the appropriate RF-signals. There is therefore a need for a RFID tag which takes account of secrecy and privacy aspects.